R1/C1 is connected to the backplate of the capsule, which is the low impedance side and being held on a steady 60V bias.It is therefore AC-connected to the ground, while DC-GND-connection is isolated by C1.This way the capsule can be polarised with +60V through R1.

So what happens if you 1)increase 2)decrease 3)remove C1?

Increase - nothing, only it would take some time to charge the polarization voltage if you'd go to extreme values.Practically there is a limit, as larger condensers have more leaking current.Pol. voltage will become lower, depending on the type of C used.Electrolytics e.g. cannot be used here.

If you decrease it, the mic will start to deliver more hum (filter becomes less efficient), finally if you remove C1 (=maximum decrease) the backplate will no longer be referenced to GND AC-wise. The AC signal circuit is no longer closed, except for some parasitic capacities of the mount.The mic will deliver very low audio level.

The THD is given by the capsule and won't change to much with external components.

Really? To my knowledge, a capsule wouldn't necessarily dominate the generation of nonlinear distortion in comparison with the electronics - except for the modern high headroom designs. An interesting paper by Peus of Neumann (1997) for example reveals that nonlinearities of pressure gradient capsules will mostly dominate (if at all) in the upper third or upper half of the frequency range - depending on capsule type but also head amp topology (tube, single FET, etc.) Among other examples, they provided a comparison of KK67/87 capsules with U67 and U87 electronics.

Now I would be really interested to learn about distortion measurements of M7 or KK47 capsules (Oliver?). On the other hand, the u47 circuitry produces a fair amount of distortion (low order, dominant k2) at relatively low sound pressure - with 0,8% THD quoted for 110dB SPL. So an M7 or KK47 would even exceed these figures?

I think distortion measurement protocols for capsules are hard to design.

Suppose you can somehow extract and eliminate THD distortion from all electronic components in the chain, including the mic processor itself, how do you want to trigger (or simulate) and isolate the distortion emanating from a capsule? With a sine wave, fed into a speaker, fed into the capsule?

That is about as far away from a real-world scenario as can be. The absence of complex wave forms, let alone the random air movement associated with a singer's exhaling, makes it really hard to understand what non-linearities come from what component. There is also the electro-mechanical interaction of the mic amp's high impedance stage coupled to the capsule.

Bottom line for me: If I hear distortion in a mic, the first thing I do is replace the capsule with a known good one (i.e. new, if available). If the distortion is no longer audible, by deduction, it was the capsule. if the distortion is still there, by the same deduction, it is something downstream from the capsule. I challenge anyone reading this to design or point to a testing protocol to isolate capsule distortion that is less Luddite.

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Really? To my knowledge, a capsule wouldn't necessarily dominate the generation of nonlinear distortion in comparison with the electronics

Distortion measurements of capsule are given by the capsule and will not change unless a nonlinear feedback circuit is designed to compensate.

The capsule of a 47, it does not matter M7 or K47, will start to distort earlier than the buffer amplifier itself, that is one of the most liked features of the 47 design.

Distortion is a funny thing, it is still measured static and yes, we do have intermodulation distortion, but it tells us nothing else than the static relation of the measured frequency at a given pressure level. A chosen set up that really does not reflect real life recording set ups.

I think distortion measurement protocols for capsules are hard to design.

Suppose you can somehow extract and eliminate THD distortion from all electronic components in the chain, including the mic processor itself, how do you want to trigger (or simulate) and isolate the distortion emanating from a capsule? With a sine wave, fed into a speaker, fed into the capsule?

A measurement setup isn't too hard to design:

1.You need a very linear buffer amplifier.No problem, as levels are high when measuring distortion; a JFet Op-Amp can be used, almost zero distortion.

2. As acoustic signal generator take two speakers coupled to an as small as possible volume.Extremely high SPL can be reached this way (imagine, a plug type headphone in your ear can easily deliver 130+ dBSL).Both speakers are feed with different tones, e.g. 5000Hz and 6000Hz.Now IM dist. (intermodulation distortion) is measured.E.g. 1st order dist. would be 1kHz, far below the 1st harmonic of the generating speakers (10kHz).

Kai,Seems as though you're just repeating the Peus idea, and not addressing Klaus' comments.If you are addressing Klaus' comment, I'm curious about how you are connecting your test to his concern. Seem like there is no link between the two.

It includes measurements of difference frequency distortion (DFD) with a setup of two speakers and the complete microphone (electronics and capsule). The basic arrangement is to feed a sweep of two sines with a relative spacing of 80Hz to one speaker each. Thus, the speakers on their own only generate THD, their two signals combine acoustically in front of the mic, and you can be sure that the DFD you measure is actually caused by the mic itself.DFD yields even and odd order distortion components: d3 (odd order) occur at 2*f1-f2 and 2*f2-f1, rising continuously with the sweep. d2 (even order) occurs at f2-f1 and remains constant. By appropriate filtering, you can exclude the THD components generated by the speakers. What Peus measured is just d2 by cutting everything except 80Hz. The problem of measuring d3 in this setup is that they're only 40Hz apart from the freq's of the input sweep, so they obviously couldn't be filtered adequately (or they didn't bother) and thus were omitted.

(...) Thus, the speakers on their own only generate THD, their two signals combine acoustically in front of the mic, and you can be sure that the DFD you measure is actually caused by the mic itself.

This test does not address the question of how to isolate capsule distortion/artefacts from amp distortion or the interaction of the two in generating unwanted distortion/artefacts.

A previous post mentioned to simply hook up the capsule to a mic amp with (high headroom) op-amp design. But that is not real-world scenario either: take the fet47 which uses the same capsule as the U47 tube. So this would be theoretically a good capsule distortion testing device, as the fet47's amp has a higher headroom than the U47 tube by at least 10dB, depending on setting.

Yet, the fet47 is known for an initial fast transient response with a lag time response to the full signal (a priced quality for kick drum application.) That means, the anomalies or characteristics of a mic amp cannot be easily taken out of the equation, even when, on paper, its static specs look flawless and suitable to isolate the capsule's flaws.

I just think the issue of isolating whether capsule or amp distort first when hit with massive SPLs may be more of a red herring upon deeper investigation, and may just leave us with this:

Testing a capsule's contribution to a mic's distortion may only be possible by substituting either capsule or mic amp with like components (after establishing a distortion base line through testing of one or several healthy specimen.) Then, one would listen for a decrease in audible distortion or other artefacts. If no decrease is audible after capsule or mic amp substitution, one can safely assume the relative level of distortion onset for that model, and, if that level is objectionable, that the mic as a whole may not be suitable for the type of recording envisioned.